1. Field of the Invention
This invention relates to overcurrent protection for power supply systems. It is particularly useful in a local area network (LAN) system where a LAN interface supplies power to other devices on the network. It is applicable to a wired LAN interface no matter what type of device the interface is used with, be it an adapter card, a bridge, a repeater, or some other device with a LAN interface port that supplies power to other devices to which it is connected.
2. Prior Art
Overcurrent protection circuits are well known in the art. Often, these circuits are very complex, requiring many components. The circuit disclosed in FIG. 3 of U.S. Pat. No. 4,404,473 to Fox is a good example of a fairly complicated overcurrent protection circuit. Prior art overcurrent protection circuits also often have the disadvantage of requiring multiple, separate power supply voltages. An overcurrent protection circuit that requires multiple supply voltages for proper operation is disclosed in U.S. Pat. No. 5,245,523 to Juzswik.
These limitations have become increasingly important, as overcurrent protection circuits in modern times are required in devices and systems where cost and space are critical considerations in design decisions. An example of a system in which cost and space are important is a local area network (LAN) in which a voltage is supplied to an interface port, which in turn powers other devices on the network through a cable. Such a LAN is described in ANSI/IEEE Standard 802.3, "Information Processing Systems--Local Area Networks," 1989, which specifies a LAN commonly known as "Ethernet" and which is incorporated herein by reference. In Ethernet, an attachment unit interface (AUI) port provides a 12 volt supply voltage over a cable to a medium attachment unit (MAU). The AUI port can be found in various Ethernet devices, such as repeaters and adapter cards for personal computers. Overcurrent protection is required at this interface, since a low impedance fault such as a short circuit in a cable or an MAU can disrupt the supply voltage to other MAUs and cause a single point failure. What is needed for this interface is a very simple overcurrent protection circuit with few components which can be operated using the 12 volt AUI supply voltage already present at the Ethernet AUI interface.